The present invention relates generally to differential measurement probes with input signal terminations and more particularly to a differential termination and attenuator network for a measurement probe having DC common mode voltage compensation.
High speed measurement systems generally feature a resistive input termination that matches the characteristic impedance of the input signal transmission line. This is done to minimize input signal reflection problems and provide the best signal fidelity. A dual 50 ohm input termination network is the most common topology for high speed differential systems. Although 50 ohm input terminations are usually connected to the measurement system ground, some applications require termination to a common mode DC termination voltage. The availability of an adjustable DC termination voltage in the measurement termination network allows control of the DC loading of the signal source, such as in a high speed serial data signal having a significant DC common mode component.
FIG. 1 is a schematic representation of a termination network 10 having an adjustable DC termination voltage VTERM used in the P7350SMA Differential Probe, manufactured and sold by Tektronix, Inc. The DC termination voltage VTERM is provided by an external source and coupled to the common node of the termination resistors 12 and 14. The termination resistors 12 and 14 are 50 ohm resistors that typically match a 50 ohm differential source impedance of a device under test. The differential input signal is coupled via SMA input connectors 16 and 18 to the termination resistors 12 and 14 and to the inputs of high impedance attenuators 20 and 22. The opposite ends of the high impedance attenuators 20 and 22 are coupled to ground. Each attenuator 20 and 22 has a resistive voltage divider consisting of resistive elements 24 and 26 coupled in parallel with a compensating capacitive voltage divider consisting of capacitive elements 28 and 30. The intermediate nodes 32 of the resistive/capacitive voltage divider networks are coupled to the inverting and non-inverting inputs of a differential amplifier 34.
The matching of the DC termination voltage VTERM of the measurement probe to the DC common mode component of the input signal allows the input signal to be directly connected to the measurement probe inputs without AC coupling by minimizing the DC loading on the signal source. The high speed differential amplifier 34 generally has a limited input dynamic range. The high impedance input attenuators 20 and 22 are provided to extend the linear dynamic range of the high speed differential amplifier 34. The DC common mode voltage at the attenuators 20 and 22 outputs results from both the common mode component of the input signal and from the common mode termination voltage VTERM of the termination network. A well designed differential measurement amplifier 34 will have a very large common mode rejection ratio, CMRR, which largely eliminates the DC common mode voltage in the amplifier output signal. Any non-zero, DC common mode voltage present at the input of the differential amplifier 34, however, will reduce the effective linear dynamic range of the amplifier 34. A large DC common mode voltage in the input signal can easily overdrive the differential amplifier 34, making it impossible to measure the often small differential mode signal.
What is needed is an input differential termination and attenuation network for a measurement probe that provides a DC common mode voltage output to the inputs of a differential measurement amplifier for optimal dynamic range of the amplifier. The input differential termination and attenuation network should monitor the input signal and provide a scaled DC termination voltage that can be adjusted to produce zero loading of the DC component of input signal source. The input differential termination and attenuation network should also provide a scaled compensation voltage that produces the optimal dynamic range for the differential measurement amplifier.